The ATG is considered suitable for custody transfer if it meets the accuracy tolerance requirements. The subsequent (periodic) verification of an ATG used in custody transfer is recommended to be inspected and have its accuracy verified at a single level on a monthly schedule. The liquid level where the ATG is verified at should be randomly chosen and should be within normal opening and closing gauge readings of the tank. The procedure for performing this verification should be the same as required for the initial verification, as specified in “API MPMS Chapter 3.1B”, except that the accuracy is verified at a randomly chosen single level, within the normal operating range of the tank.
• The measurement tape and weight used for ATG setting or verification should be a reference master tape/weight (bob) combination certified by an accredited calibration laboratory and traceable to a national measurement standard, or a working tape/weight (bob) combination that has been recently compared with a certified reference tape and weight meeting the maximum permissible error limits specified in “API MPMS Chapter 3.1A”. • If we are hired to perform initial or monthly verification, then this must be performed according to the procedures in “API MPMS Chapter 3.1 B”. A note regarding the conversion of US customary units to metric. Although API standards are required to use metric units as primary and customary units as secondary the people drafting API standards tend to think in US customary units and convert to metric. The closest metric unit to 1/16-inch is 1mm, however the closet conversion to 3/16-inch is 4 mm.
2.7.3 Error Caused by Installation and Operational Conditions The error caused by installation and operational conditions of an ATG in custody transfer service, should not be more than ± 3 mm or ± 1/8 inch.
2.7.4 Initial and Subsequent Verification
API MPMS Chapter 3.1 B requires both initial and subsequent verifications to be performed to verify the accuracy of the ATG’s. The initial and subsequent tolerance for ATG’s in custody transfer service is no more than ± 4 mm or ± 3/16 inch, when compared to manual measurements, apart from crude oil lease tanks used for custody transfer, which is no more than 6 mm or ± 1/4 inch. The initial verification of an ATG requires measurement comparisons to be made with the liquid level within regions of the tank corresponding to upper, middle and lower thirds of the tank’s working capacity. The time interval between the verification measurements at the three different levels should be kept as short as possible. The overall accuracy is verified by comparing the ATG against manual reference level measurement at three different levels and evaluating the differences between the readings.
2.7.5 Precautions
Where possible ATG’s should be read at the tank rather than in the control room as the level resolution from the transmitted signal can cause a variation in the reading. ATG’s may or may not be housed in still pipes. If they are, however, the still pipe must be slotted, as required for manual gauging. In certain locations, still pipes without slots (solid or non-perforated) have been used to comply with local air pollution regulations. Solid still pipes can lead to serious errors in level and temperature readings and should not be used for measurement.
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